In many prior refrigeration systems that employ a variable capacity screw compressor, the oil that seals, cools and lubricates the compressor, after being separated from compressed refrigerant at an oil separator into which the compressor discharges, is passed through an oil cooler in the course of being returned to the compressor from the oil separator. The oil cooler is usually a heat exchanger through which the oil flows in indirect heat exchange relation to a cooling medium. When the oil cooling medium is air, the oil cooler must include a fan or blower for producing a substantial rate of air flow across the heat exchanger surfaces. A water-cooled oil cooler requires a source of cool water or a cooling tower or the like, and usually also a pump and associated plumbing for circulating the water through the oil cooler. In all such installations a substantial first cost is involved in providing the oil cooling heat exchanger, its plumbing, and the fan, blower or pump needed for circulating the cooling medium; and there is a continuing and rather substantial operating cost for the power needed for circulation of the cooling medium.
In the refrigeration system disclosed in U.S. Pat. No. 3,710,590, to E. J. Kocher, compressor oil, after being separated from compressed refrigerant, was cooled by indirect heat exchange with liquid refrigerant drawn from a high pressure receiver. By means of a refrigerant pump, the withdrawn refrigerant was circulated through an oil cooler, thence through a desuperheating coil in the oil separator, and was finally discharged to the inlet side of the condenser to be cooled back down to saturation temperature and returned to the receiver. A principal advantage of this arrangement was that it avoided fouling of the oil cooling heat exchanger such as could occur when the cooling medium was air or water that might contain dirt. However, the system needed a heat exchanger unit for oil cooling as well as a pump for circulating the refrigerant therethrough.
In the refrigeration system disclosed in U.S. Pat. No. 3,795,117, to Moody et al, liquid refrigerant was fed directly into the screw compressor, intermediate its suction and discharge ends, to cool the compressor and the captive oil therein. This eliminated the need for an oil cooling heat exchanger, since the oil was cooled by direct heat exchange at the compressor, and it also eliminated the need for a pump because the refrigerant diverted to oil cooling flowed to the compressor under the difference in pressure between the high pressure side of the system and the compressor stage at which the diverted refrigerant was injected.
Other arrangements have also been proposed wherein refrigerant used for compressor oil cooling, drawn from the high pressure side of the system, was returned to the screw compressor at a low pressure or intermediate pressure stage thereof, to eliminate the need for a pump for circulation of the oil cooling medium.
In fact, however, such prior systems were not particularly economical in operation, and with increasing energy costs their operating inefficiencies have become more significant. When the refrigerant used for oil cooling is allowed to undergo a decrease in pressure for oil cooling purposes, then some portion of the compressor input power is being devoted to oil cooling. The power rating of the compressor must therefore be correspondingly higher than would be needed if all of its input power were being applied to the refrigeration task for which it is intended. The excess compressor power rating required for oil cooling represents a capital cost which at least partially offsets the capital saving achieved by eliminating an oil cooling heat exchanger and a pump for oil cooling medium. More important, it has been found that compression of the refrigerant used for oil cooling consumes from 3% to 12% of the full load power delivered by the compressor motor. Taking the cost of energy at the currently estimated $200 per horsepower per year, this means that in a system with a 200 horsepower compressor, the energy cost for oil cooling alone will be between $1,200 and $4,800 per year.
The present invention contemplates a screw compressor refrigeration system wherein liquid refrigerant is employed for cooling the oil that lubricates and seals the screw compressor but wherein the refrigerant so employed is both drawn from and delivered back to the system at its high-pressure side and therefore does not pass through the compressor.
The most nearly pertinent prior art with respect to this arrangement is U.S. Pat. No. 3,874,192, to E. Kato. In the system disclosed in that patent, liquid refrigerant drawn from the high pressure receptacle is delivered to the mixture of oil and compressed refrigerant flowing from the compressor to the oil separator. Such delivery takes place through an atomizer in the duct that communicates the discharge outlet of the compressor with the inlet to the oil separator. To ensure flow of liquid refrigerant through the atomizer, as the patent points out, the liquid refrigerant source must be at a higher elevation than the atomizer port, or else the atomizer outlet must comprise an ejector that utilizes suction effect due to flow of compressed refrigerant through the duct in which the atomizer is installed.
It is evident that the apparatus disclosed by the Kato patent is intended for a refrigeration system small enough to pose no problem in providing for the necessary height relationship between the liquid refrigerant source and the atomizer outlet. The Kato apparatus may be operative when that refrigerant source is at or only slightly below the level of the atomizer outlet, but it obviously could not function satisfactorily with the atomizer port at a substantially higher level than the liquid refrigerant source.
A more important objection to the Kato apparatus, and one that is not apparent from the patent disclosure, is that it is not satisfactorily operative with a variable capacity screw compressor such as would be used in a large refrigeration unit. In the conduit that carries liquid refrigerant to the atomizer outlet Kato has an electromagnetically actuated valve that is open when the compressor motor is running and closed when that motor is stopped. The output of a variable capacity screw compressor can be controllably varied from the full 100% of its capacity all the way down to as little as 10% thereof. At low compressor outputs, and with merely on-off control of the flow of liquid refrigerant for oil cooling, there would be excessive delivery of such refrigerant if the high pressure receptacle were at a higher elevation than the atomizer outlet and no delivery of it if the high pressure receptacle were substantially below the atomizer outlet. With no delivery of liquid refrigerant, there would of course be no oil cooling, with obviously undesirable consequences. Delivery of liquid refrigerant at too high a rate relative to the rate of discharge of compressed refrigerant from the compressor would have an equally detrimental effect because the compressor refrigerant would be cooled to saturation temperature, and drops of liquid refrigerant would form in the stream of mixed oil and refrigerant entering the oil separator. The liquid refrigerant would be separated out of the compressed refrigerant vapor along with the oil, and such liquid refrigerant would cause cavitation at the pump that circulates oil back to the compressor from the oil separator, so that the compressor would be starved for oil in consequence of loss of oil pressure at that pump.